1. Field of the Invention
The present invention relates to an etching process for minutely etch-machining layers deposited upon a substrate, particularly to an etching process which enables reactive ion etching suitable for the minute etch-machining to be effected efficiently with an increased etch rate.
2. Description of the Prior Art
With the minimization in size of functional devices in accordance with the recent advance of processing techniques, the necessity of forming recesses of the order of submicron in layers of semiconductors, etc. has been increasing. Particularly, it is often indispensable in fabrication of functional devices, such as Josephson devices or the like, to etch extremely minute grooves of about 0.1 .mu.m in width.
When a recess or groove with such a minute size is being formed, it is required that an aspect ratio, i.e., the ratio of the depth to the width of the recess, be as large as possible, while the side wall of the recess be made as vertically as possible. However, even in the case where such minute recesses are formed by making use of a reactive ion etching (RIE) process which generally effects an anisotropical etching wherein the etch rate in the normal direction is appreciably larger than that in the lateral direction, a difficulty is encountered such that the etch rate is extremely low as compared with that in etching of recesses having a broader opening area. As a cause for this, it is considered that the smaller the width of the recess and the larger the aspect ratio, the more difficult will it be for gaseous reaction products produced by etching to move out of the recess, which interfere with the progress of etching.
Further, in RIE, since the etching is effected anisotropically by means of positively charged ions which have been accelerated by direct voltage (self-bias voltage) between electrodes and are incident perpendicularly upon the layer, it is necessary to increase the self-bias voltage in order to augment the etching anisotropy. However, in RIE, the increase of the self-bias voltage requires an increase of the radiofrequency power which is accompanied with a substrate temperature elevation. Organic resists to be used as a masking material against etching are usually deteriorated at considerably elevated temperatures, so that in the case where a long time etching is required for deepening the recess, the self-bias voltage cannot be so much increased in order to suppress the temperature elevation, and a sufficiently augmented etching anisotropy is, therefore, not achievable.